1. Field of the Invention
This invention relates to a camera provided with focus detecting means having a plurality of distance measuring fields and for detecting the imaged states of object images having a variation occurring in the relative spacing therebetween on the basis of the photoelectric conversion outputs of the object images in conformity with the imaging state of an imaging optical system, and distance measurement propriety judging means for judging on the basis of the output from exit window output means whether the distance measuring light beam of each distance measuring field of said focus detecting means is eclipsed by the imaging optical system.
2. Description of the Related Art
Heretofore, in a lens interchange type camera, or camera system, provided with a focus detecting device of the phase difference detection type on the camera body side, for example, the fully open F number information of a photo-taking lens has been transmitted from the photo-taking lens to the camera body to thereby judge the presence of the eclipse of a distance measuring light beam and judge whether the focus detecting device is operable.
However, a popular photographic photo-taking lens has non-paraxially two exit windows (exit pupils) as is suggested by the fact that a decrease in a pupil area called aperture eclipse occurs at the image plane position off the optical axis. Accordingly, for a distance measuring field off the optical axis of the photo-taking lens, it is difficult to judge simply from the fully open F number information whether a distance measuring light beam is included in a photographing light beam. So, in the construction of the focus detecting device, it has been necessary to consider the exit windows of all photo-taking lenses mounted and set the distance measuring light beam so that an eclipse may not occur.
However, with the recent downsizing of photo-taking lenses, the location of the exit window has a strong tendency to become close to the predetermined imaging plane and on the other hand, with the higher magnification of zoom ratio, the location of the exit window has a tendency to become far from the predetermined imaging plane. In addition, the distance measuring field of the focus detecting device which has heretofore been only on the optical axis of the photo-taking lens has also come to be disposed off the optical axis and it has become more severe to ensure the absence of the eclipse of the distance measuring light beam for all photo-taking lenses and all distance measuring fields as described above.
In a camera having a conventional focus detecting device, a distance measuring field frame has been integrally formed on a focusing screen or a liquid crystal display element has been disposed adjacent to the focusing screen, and this liquid crystal display element has been electrically driven to thereby display the location of the distance measuring field over the object image within a finder field.
Now, as previously described, a popular photographic photo-taking lens has non-paraxially two exit windows as is suggested by the fact that a decrease in pupil area called aperture eclipse occurs at the image plane position off the optical axis. With the recent downsizing of photo-taking lenses, the location of the exit window has a strong tendency to become close to the predetermined imaging plane and on the other hand, with the higher magnification of zoom ratio, the location of the exit window is in a tendency to become far from the predetermined imaging plane.
In conformity therewith, in a lens interchange type camera (system) provided with a focus detecting device of the phase difference detection type on the camera body side of a photo-taking lens, for a distance measuring field lying off the optical axis of the photo-taking lens, it has become necessary to judge the presence of the eclipse of a distance measuring light beam, discriminate whether the focus detecting device is operable, and suitably render a particular distance measuring field inoperative.
However, in the conventional distance measuring field display, the operative states of respective distance measuring fields are not displayed, and this has led to a problem that the photographer cannot recognize which distance measuring field can be used.